The present invention relates generally to communications systems, and more particularly, to a spectrum allocation for communication systems.
In this communication age, content providers are increasingly investigating ways in which to provide more content to users as well as interfacing with a larger group of users. The latest communications systems use digital broadcast satellites to broadcast to users in a forward direction. System operators have recognized that user requests are relatively small in terms of data compared to the amount of data broadcast from the satellite. Thus, for example, in the DirecTV(copyright) system, user requests are transmitted through telephone lines and programming is provided to the users via satellite.
In addition to satellites, stratospheric platforms are currently under development. One such stratospheric platform is Helios that is being developed by AeroVironment. The Helios stratospheric platform is an unmanned vehicle that can fly for several months at a height of about 60,000 feet. Helios is a solar powered electric plane that is modular in design and may be configured to carry a variety of payloads. Stratospheric platforms have numerous advantages over geo-stationary satellites, including that a large bandwidth density can be projected over a small but populated area, associated transmission delays are significantly reduced, the power required for transmitting and receiving is substantially smaller, and the user elevation are higher in general. Also, these stratospheric platforms can be deployed relatively rapidly compared to satellites and thus, if a business need increases, the system capability may be increased quickly through deploying new platforms.
Commonly, such communication systems have a high altitude communications device such as a satellite or a stratospheric platform as described above. Also, such systems have user terminals and a gateway station or plurality of gateway stations that communicate with the high altitude communications device and link the user terminals to terrestrial networks.
Because the frequency resources are scarce for over-air transmissions, various multiplexing schemes are used to provide a greater number of communication signals within an allocated communication ban. Such schemes include code division multiple access (xe2x80x9cCDMAxe2x80x9d), time division multiple access (xe2x80x9cTDMAxe2x80x9d), frequency division multiple access (xe2x80x9cFDMAxe2x80x9d), or combinations of these schemes. Typically, the user links between the high altitude device and user terminals operate at different frequencies, times or codes than those of the feeder link to prevent interference. However, because frequency resources are scarce, it would be desirable to provide a system that utilizes the same communication frequency spectrum of the user links in the feeder link.
It would therefore be desirable to provide a communication system that efficiently uses the available spectrum for both feeder and user links simultaneously.
It is an object of the invention to provide a communication system in which the feeder link does not require extra communications frequency spectrum from those of the fixed user links.
In one aspect of the invention, a communication system has a high altitude communication device generating a plurality of user link beams having a first communication characteristic and a feeder link having a first communication characteristic. A gateway terminal, with higher gain and narrower beam-width receives not the user link but a feeder link beam with a much higher data throughput rate than those from a user link. A user terminal receives at least one of the user link beams. The communications system features a xe2x80x9cspoke-and-hubxe2x80x9d (S-and-H) architecture. Connectivity among users is established not directly but through a central hub. A platform may be connected to various users via different user link beams and codes. But there is no cross-beam nor cross-code connectivity on board the platform. The platform pipes all the user signals back to a ground hub through a high data rate feeder link. The connectivity among user signals is achieved through switching and/or routing mechanisms on ground. The plurality of user link beams comprises a first user link beam associated a first cell and a first isolation zone outside said first cell. The plurality of user link beams comprises a second user link beam having the first communication characteristic and positioned within a second cell and a second isolation zone outside said second cell. The second isolation zone overlaps the first isolation zone. The feeder link is positioned within the first isolation zone and the second isolation zone. The feeder link has the first communication characteristic.
In a further aspect of the invention, a method of operating communication system comprises the steps of:
generating a first user link beam having a first communication characteristic and a first cell zone and a first isolation zone;
generating a second user link beam having a first communication characteristic and a second cell zone and a second isolation zone at least partially overlapping said first isolation zone;
generating a feeder link beam having the first communication characteristic and positioned within said first isolation zone and said second isolation zone.
One advantage of the invention is that the user link transmission antenna and the feeder link transmission antenna are decoupled to allow independent optimizations on the platform use link and feeder link antenna designs.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.